Global and local geospace modeling in ISTP

The objective of the University of Maryland ISTP theory project is the development of the analytical and computational tools, which, combined with the data collected by the space and ground-based ISTP sensors, will lead to the construction of the first causal and predictive global geospace model. To attain this objective a research project composed of four complementary parts is conducted. First the global interaction of the solar wind-magnetosphe re system is studied using three-dimensional MHD simulations. Appropriate results of these simulations are made available to other ISTP investigators through the Central Data Handling Facility (CDHF) in a format suitable for comparison with the observations from the ISTP spacecrafts and ground instruments. Second, simulations of local processes are performed using a variety of non-MHD codes (hybrid, particle and multifluid) to study critical magnetospheric boundary layers, such as the magnetopause and the magnetotail. Third, a strong analytic effort using recently developed methods of nonlinear dynamics is conducted, to provide a complementary semi-empirical understanding of the nonlinear response of the magnetosphere and its parts to the solar wind input. The fourth part will be conducted during and following the data retrieval and its objective is to utilize the data base in conjunction with the above models to produce the next generation of global and local magnetospheric models. Special emphasis is paid to the development of advanced visualization packages that allow for interactive real time comparison of the experimental and computational data. Examples of the computational tools and of the ongoing investigations are presented.     

The elemental composition in energetic particle events at high heliospheric latitudes

The relative abundances of low energy ions (0.6–2.0 MeV/n) in solar energetic particle (SEP) and corotating interaction region (CIR) events have been measured by the EPAC experiment aboard Ulysses since launch in October 1990 until the present time. We give an overview of the abundances of heavy ions (He, C, Ne, Fe) relative to oxygen during energetic particle events lasting longer than 5 days during the in- and out-of-ecliptic phase of the mission. While the period Oct. 1990 to Aug. 1992 was dominated by high solar activity the Ulysses out of ecliptic passage at solar latitudes up to 45° went parallel to the declining phase of solar activity. Thus a very clear structure of corotating interaction regions was observed. While the in-ecliptic composition is in general agreement with measurements made near the Earth, the development of the CIR-composition shows two phases: From Aug. 1992 to May 1993 the C/O-ratio is 0.55–0.70, afterwards it increases to 0.8–0.9. This increase is correlated to the disappearance of the current sheet at 30° solar latitude reported by Smithet al. (1993).     

EXOSAT observations of the iron line emission of Her X-l during a 35 day cycle

In March/April 1984 eleven EXOSAT observations of Her X-1 were performed sampling a full 35 day cycle. Spectral analysis of the ME and GSPC data shows that the iron line emission is present during all phases. During the main-on state we see an iron line at 6.65 ± 0.07 keV with a FWHM of 1–2 keV and an equivalent width of 300 to 400 eV. The high resolution GSPC data indicate that the line profiles have external wings and are not simple Gaussian. We report for the first time on the detection of an iron line during the intermediate-on state with about the same parameters as the main-on state line but an equivalent width a factor of 2 larger. During the off state between main-on and intermediate-on we detected a broad iron line feature at about 6.0 keV with an equivalent width of 2 keV. We discuss the Alfven region and a hot corona at the inner region of the accretion disk as the possible sites of the line production.     

Effectiveness of Airborne Integral Relay Operations

"Integral relay" is a relay concept in which all the aircraft in a control sector that are under the control of an Army flight operations center (FOC) are electronically capable of functioning as relays for communication with aircraft out of direct contact with the FOC. This paper deals with the effectiveness of integral relay operations based on the assumption of random spatial distribution of aircraft. It describes the development of a control sector line-of-sight (LOS) model and of a methodology for effectiveness analysis, and the application of the methodology to one numerical example. It is shown that the LOS conditions in the control sector may be represented by two LOS characteristics which are functions of both aircraft clearance and the terrain and FOC characteristics. Probabilities of direct and indirect (via relay) LOS between the FOC and randomly located aircraft are derived for operations without and with integral relay. The effectiveness, expressed as an increase in LOS probability, is a function of aircraft density and the above two LOS characteristics and is representative of the control sector as a whole. Numerical results are obtained by probability analysis and statistical sampling.     

Terminal Guidance for Impact Attitude Angle Constrained Flight Trajectories

The design of a suboptimal terminal guidance system for reentry vehicles with a constraint on the body attitude angle at impact is studied. Permissible range of the miss distance and the body attitude angle at impact is specified. The problem is formulated as a linear quadratic control problem. The Riccati equation is derived to provide time-varying feedback gains. The desired scheme is suboptimal. The region of initial states for which the system meets the specifications becomes smaller as the initial height of the reentry vehicle at initial time is decreased.     

Dual spacecraft observations of energetic particles in the vicinity of the magnetopause,bow shock,and the interplanetary medium

This article presents some of the new and important particle features that have been detected in the energy range 1 keV to 290 keV by the ISEE-1 and -2 spacecraft near the magnetopause, bow shock, and the interplanetary space. Only examples of data from the first few orbits, when the spacecraft were on the front side, are shown.Paper presented at 13th ESLAB Symposium, Innsbruck, Austria (June 5, 1978).     

Optimum Design of RC Snubbers for Switching Regulators

The optimum design of an RC snubber to suppress the surge voltage across the transistor in a switching regulator with a two-winding reactor is presented. Analyzing the surge voltage by means of high-frequency equivalent circuits, we obtain the third-order characteristic equation. This third-order equation is first analyzed by the aid of the root locus method. As a result, the region where the surge voltage can be suppressed is described in the R-C plane. Then considering the snubber loss, the optimum resistance and capacitance can be obtained. Second, the precise design procedure of RC snubbers is discussed by normalization and numerical calculations. This procedure is summarized in easy-to-use nomographs.     

A Rader Clutter Model: Average Scattering Coefficients of Land, Snow, and Ice

A model has been developed for average radar backscatter from terrain based on recent carefully controlled wide-bandwidth measurements of vegetation, snow-covered ground, and sea ice and on a comparison with measurements over North America by the Skylab S-193 scatterometer. The models for the thiree cases take the form ?° dB = A + B? + Cf+ Df?, 20° ? angle of incidence ? 70°, where the constants vary depending on polarization and terrain class. They also differ above and below a critical frequency (6 GHz for general terrain, 8 GHz for sea ice, and between 8 and 12 GHz for snow). For angles of incidence of 0° (vertical) and 10°, the model is of the form ?° dB = M(?) + N(?)f over the range 1 to 18 GHz. Hundreds of thousands of measurements contributed to the general (vegetated terrain) model, and smaller numbers contributed to the snow and sea ice models. Since 1974 all measurements have been made with University of Kansas microwave spectrometers. A brief discussion of fading shows that insufficient data are available to describe the ranges adequately.     

Scanning Spaceborne Synthetic Aperture Radar with Integrated Radiometer

Spaceborne synthetic aperture radar systems are severely constrained to a narrow swath by ambiguity limitations. Here a vertically scanned-beam synthetic aperture system (SCANSAR) is proposed as a solution to this problem. The potential length of synthetic aperture must be shared between beam positions, so the along-track resolution is poorer; a direct tradeoff exists between resolution and swath width. The length of the real aperture is independently traded against the number of scanning positions. Design curves and equations are presented for spaceborne SCANSARs for altitudes between 400 and 1400 km and inner angles of incidence between 20° and 40°. When the real antenna is approximately square, it may also be used for a microwave radiometer. The combined radiometer and synthetic-aperture (RADISAR) should be useful for those applications where the poorer resolution of the radiometer is useful for some purposes, but the finer resolution of the radar is needed for others.